This invention relates to a flame detector, and more particularly, to a testing circuit used with such a detector. It is important to monitor many burners to determine whether there are flames or not by detecting the light variation of the flames. An abnormal condition will occur if the flame is extinguished and a photo sensor or an inner circuit of the flame detector is out of order. The abnormal condition is a spouting of unburned fuel from the burner which is not producing a flame. If the flame detector is out of order, despite the fact that no flame is being produced by the burner, the flame detector outputs a signal which permits fuel to continue to be provided to the burner. This situation is very dangerous. If the flame detector operates normally, it detects whether there is a flame from the burner or not and outputs a signal to stop providing one fuel to the burner, if there is no flame. Accordingly, it will prevent the abnormal condition under which fuel is wrongly provided to the burner.
Recently, many burners may be used in a larger boiler. An erroneous operation of the flame detector in such a case may produce a disastrous result.
The operation of the flame detector itself, therefore, must be monitored. Referring to FIG. 1, a previously known flame detector is shown, including a pivoted photo shielding plate 11 which may be interposed between a flame 12 from a burner 13 and a photo sensor 14 for detecting the light intensity of the flame 12. The photo shielding plate 11 blocks off the flame 12 and presents to the photo sensor 14 the same situation as if the flame had gone out. By blocking off the flame 12, the shield 11 can confirm that the flame detector is operating normally. If the shielding plate 11 properly blocks off the flame, an amplifier 15 transmits an amplified signal from the photo sensor 14 to a detecting circuit 16. The detecting circuit 16 reacts as if there is no flame. In the example of the pivotable shielding plate, the shielding plate 11, a rotary solenoid 17 and a power source 18 are used. A switch 19 is provided between the rotary solenoid 17 and the power source 18. When the switch 19 is closed, the shielding plate 11 moves to block off the flame 12.
However, it sometimes happens that the pivoted portion of the rotary solenoid 17 is out of order. Additionally, the plate 11 may become warped or otherwise defaced so that it does not completely block the light of the flame 12. This leakage of the light causes an error in the monitoring of the flame detector. A further problem arises when the rotary solenoid 17 pivots the shield plate 11 and an electric noise results which adversely affects other electronic circuits. Furthermore, the rotary solenoid 17 needs a power source 18 for driving it and a connecting conductor between the solenoid 17 and the source 18 through the switch 19.